1. Field of the Invention
The invention relates to a method and a circuit for carrier control in a quadrature demodulator.
2. Description of Related Art
FIG. 3 shows an example of a circuit arrangement for carrier control in a quadrature demodulator. A complex-value input signal I and Q is provided to a mixer 1 for mixing with a mixing frequency fm to create a mixed signal Ir, Qr. This mixed signal Ir, Qr is input to a processing section 2 with a channel equalizer, whereby a signal delay and a so called idle time z.sup.-d is created between one of the processed signals Ie, Qe and the mixed signal Ir, Qr. The processed signal Ie, Qe is applied to a rotator 4, which rotates the input signal and provides it to a circuit for phase measurement, which is designed for the measuring of a phase .phi.m of the processed signal Ie, Qe. A controller 4 determines the mixing frequency fm and inputs the frequency to a mixer 1.
In such arrangements, a general problem occurs in the case of digital demodulators for terrestrial signals, which are usually equipped with a channel equalizer. The group delay time of the channel equalizers creates, together with the impulse shaping filters used for the carrier control and for the timing control, a substantial idle time, which essentially results in a slow control. This creates major problems for adaptation of channel equalizers.
When one looks at this problem, one can infer from the known type of circuit shown in FIG. 3 that the complex-value processed signal Ie, Qe can be further supplied to a rotator 5, which rotates the supplied signal and furnishes it to a discriminator 6 via a phase measurement stage 3.
The discriminator 6 carries out a discriminating process in a known manner and outputs the decision signal to a second controller 7 and to a back rotator 8. In addition, the measured phase φm is input to the second controller 7. The second controller 7 provides a generated signal that is dependent on the signal input to the second controller 7 to the controller 4 as the control variable. The signal of the second controller 7 is also applied as a control variable to the rotator 5 and to the back rotator 8. The controller 4 is adapted to the second controller 7 with a slower time constant. The back rotator 8 determines an error signal es, which is input to the channel equalizer 2. With such an arrangement, a second, faster control circuit is implement with a smaller idle time at the output, which is active only during a slow switching-on transition of the main controller, and which makes it possible to achieve a correct adjustment of the channel equalizer 2 more quickly.
A disadvantage of a solution using an additional, second controller is the additional expense, which consists of two additional rotator blocks and a second controller. Another disadvantage is that the actual carrier/symbol cycle is still slow.